Furnace



March 24, 1936.

E. H. BERRYMAN Er A1.

FURNACE Filed Oct. 18, l933 5 Sheets-Sheet l.

I NVENTOR S, H. .19mm/MAN.

A TTORNE Y March 24, 1936.

E. H. BERRYMAN El' AL FURNACE Filed Oct. 18, 1935 INI/ENTORST 5 Sheets-Sheet 2 .FURNACE Filed Oct. 18, 1935 5 Sheets-Sheet 3 L/oa Flz' g. l

MJI/ENTORST ATTORNEY Patented Mar. 24, 1936 UNITED STATES PATENT OFFICE FURNACE California Application October 18, 1933, Serial No. 694,102

5 Claims.

This invention relates to electric furnaces and in particular to electric muflles such as used in dentistry.

For economy of operation a muiile should be only large enough to accommodate the material being heated, but inasmuch as a dentist or other technician finds it necessary to make castings, burn out wax matrices and bake porcel-ain objects which vary to a considerable extent in size, either a large furnace must be used or a number of furnaces of different sizes. For obvious reasons the latter alternative is prohibitive for the majority of operators.

Furthermore for some purposes high temperatures are required and for others it is essential that low temperatures be used. For example in making porcelain castings a low temperature oven is first used to set the material and then a high temperature oven is used for baking and glazing the casting. After this has been done the casting should be gradually cooled without chilling to properly anneal the material and consequently the low temperature furnace must again be resorted to.

One of the objects of our invention is the provision of a multiple muflle having a set of resistance elements in common which can be used as a rheostat for either muiiie by connecting it in series with the individual resistance elements of each mule -as desired.

Another object of our invention is the provision of a mufe having a set of resistance elements a portion of which may be used as a rheostat for controlling the heat generated within the muflle.

Still a further object of our invention is the provision of a multiple furnace comprising a pair of nested mules, one being a high temperature mule and the other a low temperature muiile.

The invention possesses other advantageous features, some of which with the foregoing, will be set forth at length in the following description, where we shall outline in full that form of our invention which we have selected for illustration in the drawings accompanying and forming a part of the present specication. In said drawings, we have shown one form of our invention, but it is to be understood that we do not limit ourselves to such a form, since the invention as set forth in the claims, may be embodied in a plurality of forms.

Referring to the drawings:

Figure 1 is an isometric drawing of the assembled muilie.

(Cl. 21B-35) Figure 2 is a vertical midsection taken longitudinally of the furnace.

Figure 3 is a wiring diagram of the multiple furnace as shown in Figure 2.

Figure 4 is a wiring diagram of a modified furnace wherein a single mufile is used and in which a part of the resistance element serves as a rheostat for controlling the temperature within the muffle.

As clearly shown in Figures 1 and 2 the furnace comprises a base I on which is mounted a low temperature muflle 2. Within the muiile 2 is a second substantially smaller inutile 3 capable of operating at high temperatures. The interior side walls of both mules are provided with grooves or channels 4 and 5 within which the resistance elements of the muflles are accommodated. As shown in Figure 2 the base 6 of the mufile 2 is slightly spaced from the base I of the furnace in order to form a slot 'I within which a rheostat control bar 8 is adapted to operate. The central part of the control bar 8 is pivoted on the pin 9 secured to the base I and its inner end is adapted to register with any one of the spaced contact points II through 20, as diagrammatically shown in Figure 3. To the outer end of the control bar 8 is secured a knob 2| of insulating material and the base I is provided with spaced reference marks 22 arranged in an arc and corresponding to the contact points II through 20. By placing the control bar 8 in registration with any one of the reference marks 22 the opposite end of the control bar will contact one of the corresponding contact points II through 20.

Disposed within the grooves or channels 4 provided in the inner walls of the muiile 2 are two sets of resistance elements. As shown in Figure 3 one set consists of resistance elements 3I through 4I connected in series and which may conveniently be located in the upper portion of the mufile 2. In effect all of these elements form a single resistance, and since the muille 2 is designed to be operated at low temperatures (-1400 F.) these resistance elements may be made of a base metal wire such as nichrome. The other set consists of resistance elements 42 through 5U connected in series and preferably located in the lower portion of the mule 2. Each of the elements 42 through 50 is connected as shown in Figure 3 with the contact points II through 20. By rotating the control bar 8 any one of the contact points II to 2U can be connected with the line or source of current and consequently the resistance elements 42 through vportion of the large muiiie 2.

may be made to serve as a rheostat. Disposed within the small high temperature muie 3 is a resistance element 5| one end of which is secured to the lower end of the resistance element 3|, While the other end is connected to the upper terminal 52 of a double throw switch 53. The element 5| should be designed to operate at a temperature 2450 F. or higher and therefore may be made of platinum wire. element 40 connects with the opposite terminal 54 of the switch 53 while the switch 53 connects with the upper end of the element 4|. The lower end of the element 3| connects through a switch with the remaining leg of the line, thereby completing the circuit. By closing the switch 55' and the switch 53 against the terminal 52 the rheostat formed by the elements d2 through 50 may be placed in series with the resistance element 5| of the small muflie 3, and the temperature adjusted by cutting in as many of the resistance elements 2 through 5i! as is desired. By closing the switch 53 against the terminal 54 the rheostat may be placed in series with the resistance element 3| through 50, located in the upper It is to be particularly noted that the iloor of the muiile 2 also serves as a oor for the muiile 3, and that the resistance elements forming the rheostat are located in this oor common to both mufles. In either'position of the switch 53 the current must of necessity ow through the rheostat, and consequently heat the floor of both muies. If therefore it is desired Vto make a small casting the switch 53 may be closed against the terminal 52 in which case the small muiile is heated while the large muflie is inactive, but nevertheless serves as additional insulation. If it is desired to make a large casting the switch 53 is closed against the terminal 54 whereby the large muiile 2 is heated and the small muille 3 is inactive. In this case the casting may be placed either on top of the small muffle or immediately in front of it on the common floor. As shown in Figures l and 2 the furnace may be provided with a pyrometer if desired. Y

Furthermore it will be seen that this arrangement of muiiles can be used very eifectively for two purposes. The upper portion 2 being a low heat mufe serves as a preliminary drying charnber and the lower portion 3 as a high heat iinishing chamber or muiile.

While the upper muiile heats to the desired vtemperature the lower mufe is automatically heated by means of conducted heat without the use of its heating elements being operative thereby saving time and electrical pow-er.

After completing a high heat baking or fusing process in the lower muflle the work is'transferred to the upper muiile for annealing. The temperature of the upper muiile is high enough for this purpose because some of the original heat is conserved within its well insulated outer Walls and also from the conducted heat of the lower muiile.

It should also be noted that in transferring the casting, porcelain jacket crowns or other dental work from one muiile to the other there is no necessity of taking it out of the furnace and sub- 'jecting it to the outside atmospheric temperature, and consequently it is not chilled and strains are prevented.

The lower end of the In Figure 4 we have shown a wiring diagram for a furnace in which a single muiile is utilized. In this modication there are two sets of resistance elements, one set consists of resistance element El through 69, while the other set consists of resistance element 1| through 8|, and as clearly shown the elements of one set are disposed intermediate the elements of the other set. Each of the elements 5| through 69 are connected with the spaced contact points 82 through |01. The control bar |02 connects through the switch |53 with one leg of the line, while the other leg of the line connects with the left hand end of the element 8|. Contact lill connects both with the left hand end of the resistance element S9 as well as with the right hand end of theresistance element ll. It will therefore be noted that by closing the switch |53 and placing the control bar |02 in contact with contact IBI the current is made to pass successively through resistance element 7H through 8| and that by ro.- tating the control bar |02 to the left any desired number of elements 6| through 59 may be placed in series with the elements 'H through 8|.

It will therefore be seen that we have provided a furnace comprising a pair-of muiiies so associated that they supplement each other as to temperature as well as to capacity or size.

We claim:

l. An electric furnace comprising a pair of muies of substantially different sizes, one contained within the other and having a wall in common heated by a set of resistance elements which function as a rheostat for either mufe.

2. An electric furnace comprising a pair'of muiiles having a wall in common; heating elements disposed within each muie; a set of resistance elements disposed within said common wall forming a rheostat and a switch for connecting said rheostat in series with the heatingv element or" either muiiie.

3. An electric furnace comprising: a muile; a second substantially smaller muiiie disposed Within said rst muie; a set of resistance elements within said first muiile forming a rheostat; a heating element within said second muiile; a heating element within said rst muiiie and a switch for connecting said rhecstat in series either with the heating element in the second mufle or with the heating element in said rst muflie.

4. An electric furnace comprising a base; a mufile mounted on said base; a second substantially smaller muiile formed within said first muiile; aV heating element disposed within said rst muiiie; a heating element disposed withinV saidrsecond inutile; a rheostat -disposed within one of said muiiles and a switch for connecting said rheostat in series with either of said heating elements.

5. A furnace comprising a pair of nested muiiies having a wall in common; ahigh temperature heating element associated with one of said mules; a low temperature heating element associated with the other of said muiiles; a rheostat associated with said common wall and a switch for connecting said rheostat in series with either of said heating elements as desired.

ELMER H. BERRYMAN. HENRY K. HUPPERT. 

